Furnace Blowing Cold Air in Palo Alto, CA

Furnace Blowing Cold Air in Palo Alto

A Palo Alto furnace that runs while the registers stay cold is circulating air the burners never warmed. On the forced-air gas systems in Palo Alto's Spanish revivals, Mediterraneans, and newer infill, the cause is almost always an igniter or flame sensor problem keeping the burners from staying lit. It is one of the cheaper failures relative to the equipment, even on a high-end system.

Palo Alto's marine-influenced climate is among the mildest in the South Bay, so furnaces here run lightly and intermittently. That is precisely how a cracked igniter or a carboned sensor stays hidden until a foggy cold morning when the unit cannot get the burners to hold. If the furnace heated fine last winter and now blows cold, look at a single part before the whole system.

Palo Alto owners tend to want a clean, code-correct diagnosis rather than a parts-cannon approach, and that is how we work. On Eichlers, where the original radiant-slab heat has often been retired or supplemented with ductless or retrofit forced air, we look at whatever system is actually installed rather than assuming a standard furnace.

Common causes

Cracked hot surface igniter. The most common no-heat cause. The silicon-carbide element fails on thermal cycling, so even a lightly used Palo Alto furnace wears one out. We watch a startup to confirm it never glows and test continuity, then replace it. Typically $200 to $350.

Dirty flame sensor. Burners that ignite and then cut out within seconds usually mean the flame sensor is not proving flame. Carbon on the rod is the culprit, and the board shuts gas off as a safety response. Cleaning often restores it; a worn rod gets replaced, generally $150 to $200.

Thermostat fan on ON instead of AUTO. With a smart or zoned thermostat set so the fan runs continuously, the blower pushes mild air between heat cycles and it reads as cold air at the registers. We check the setting first and confirm the system heats on a real call before opening anything.

High-limit trip from airflow restriction. A clogged filter or a restricted return overheats the heat exchanger and the high-limit cuts the burners while the blower runs. We check filter and static pressure, clear the restriction, and verify the limit resets and holds through a full cycle.

Condensate blockage on a high-efficiency unit. Many newer Palo Alto installs are 90%-plus condensing furnaces. A plugged condensate drain or trap trips the pressure or float safety and stops the burners, blower still running. We clear the drain, confirm the trap flows, and test that the safety lets the unit fire.

Gas supply or valve fault. If the igniter glows and nothing lights, we meter gas to the valve and confirm it opens on command. We test before condemning a gas part, never replace one on a hunch.

How we diagnose it

Identify the actual installed system first, since Eichler and retrofit homes are not always a standard furnace.
Run a full startup and watch where it fails: igniter, burner light, flame proven, blower engage.
Check the thermostat fan setting and confirm real heat on a call for heat.
On condensing furnaces, check the condensate drain and trap for a blockage tripping a safety.
Test igniter and flame sensor, and inspect filter and airflow to rule a high-limit trip in or out.

$75 diagnostic, credited toward any repair over $200. You get a written quote before any work begins.

Furnace Blowing Cold Air in Palo Alto: common questions

Do you serve Palo Alto from the East Bay, and how quickly?

Yes. We are based in San Ramon and cover the Peninsula and South Bay including Palo Alto, Menlo Park, and Los Altos. Same-day is best effort, not guaranteed, and we give you the honest next available slot when you call (925) 999-4095.

My Palo Alto home has a high-efficiency furnace. Does that change why it blows cold?

It adds one cause: condensing furnaces drain water, and a plugged condensate line or trap trips a safety that stops the burners while the blower runs on. On a standard furnace it is more often the igniter or flame sensor. We check the condensate path on any 90%-plus unit before assuming an ignition part.

The furnace runs but the air feels cold. Is it the whole system?

Almost never. A running blower means the motor and controls have power, so the failure is downstream. On a condensing unit we check the condensate path first, since a tripped float or pressure safety stops the burners cold. Otherwise it is usually the igniter or flame sensor, a fan left on ON, or a high-limit trip. We confirm by watching a startup, not by guessing.

Nearby and related

Furnace Blowing Cold Air near Palo Alto: Menlo Park · Los Altos · Mountain View .

This is usually a furnace repair in Palo Alto job. See our furnace repair overview or the Palo Alto service area.